1. Field of the Invention
The present invention relates to valve spool valves in which pressure applied to a port causes the position of the valve spool to change, thereby opening or closing a fluid path; and in particular to such valves that have two electrically selectable setpoints that vary a pressure threshold which must be exceeded for the valve spool to change position.
2. Description of the Related Art
Pressure operated valves are well known. Pressure occurring at a port of the valve applies a force to a valve element, such as a valve spool or poppet. Often that pressure force is opposed by a spring force, commonly referred to as the setpoint of the valve, which defines the pressure threshold that must be exceeded for the valve to operate. When the pressure force is greater than the spring force, the valve element moves from a first position to a second position. That movement opens or closes a fluid path through the valve. Thereafter, when the pressure force decreases below the spring force, the valve element returns to the first position.
Valves of this type are used to control application of fluid pressure to another hydraulically operated device in response to the level of the pressure applied to the valve. It is desirable in some applications to design a valve with two pressure setpoints that can be selected depending on operating conditions of the machine in which the hydraulic system is incorporated.
Solenoid operated valve spool valves also are well known for controlling the flow of hydraulic fluid. In a typical valve of this type, such as shown in U.S. Pat. No. 7,007,925, a solenoid actuator has an armature that moves in response to an electromagnetic field that is created by electric current flowing through a coil. The armature either directly engages or drives a pin that engages a valve spool. In response to the force imparted by the solenoid actuator, the valve spool slides within a bore into which a plurality of ports open. A typical valve of this type has three ports along the length of the bore, a supply port and a tank port with a workport in between that leads to a device being controlled by the hydraulic fluid flowing through the valve. A groove in the outer surface of the valve spool provides a path between the workport and either the supply port or the tank port depending upon the position of the valve spool within the bore. A spring biases the valve spool into a predefined position when the solenoid is de-energized.
Although the primary forces that cause the valve spool to move are produced by the solenoid actuator and the spring, it is not uncommon that surfaces of the valve spool are specifically designed so that fluid pressures acting thereon aid in moving the valve spool in one or both directions within the valve bore.